corresponding one of the arteries; and the internal
coat is serous, and analogous to that of those vessels. The veins belong
to the three following classes: (1.) The systemic veins, which bring the
blood from different parts of the body and discharge it into the vena
cava, by means of which it is conveyed to the heart; (2), the pulmonary
veins, which bring the arterial, or bright red blood from the lungs and
carry it to the left auricle; (3), the veins of the portal system, which
originate in the capillaries of the abdominal organs, then converge into
trunks and enter the liver, to branch off again into divisions and
subdivisions of the minutest character.

The _Capillaries_ form an extremely fine net-work, and are distributed
to every part of the body. They vary in diameter from 1/3500 to 1/2000
of an inch. They are so universally prevalent throughout the skin, that
the puncture of a needle would wound a large number of them. These
vessels receive the blood and bring it into intimate contact with the
tissues, which take from it the principal part of its oxygen and other
elements, and give up to it carbonic acid and the other waste products
resulting from the transformation of the tissues, which are transmitted
through the veins to the heart, and thence by the arteries to the lungs
and various excretory organs.

The blood from the system in general, except the lungs, is poured into
the right auricle by two large veins, called the superior and the
inferior _vena cava_,' and that returning from the lungs is poured into
the left auricle by the _pulmonary veins._

During life the heart contracts rhythmically, the contractions
commencing at the base, in each auricle, and extending towards the apex.

Now it follows, from the anatomical arrangement of this organ, that when
the auricles contract, the blood contained in them is forced through the
auriculo-ventricular openings into the ventricles; the contractions then
extending to the ventricles, in a wave-like manner, the great proportion
of the blood, being prevented from re-entering the auricles by the
tricuspid and mitral valves, is forced onward into the pulmonary artery
from the right ventricle, and into the aorta from the left ventricle.

When the contents of the ventricles are suddenly forced into these great
blood-vessels, a shock is given to the entire mass of fluid which they
contain, and this shock is speedily propagated along their branches,
being known at the w